In a known method of weld-seam tracking (see German laid-open application No. 24 21 191) a probe or sensor, disposed ahead of a welding torch, measures the relative positon of the torch and the two workpieces to be welded whereupon the line of their contact, i.e. the zone of the weld seam, is calculated.
A drawback of this method lies in that in the case of a change of the configuration of the weld seam or in the inclination of the torch it is necessary to change also the position of the position sensor. Moreover, the method can be applied only in the case of angle joints or when workpieces with beveled edges are to be welded. Another drawback lies in that the correct guiding of the position sensor requires it to have at least one additional degree of mobility with respect to the welding torch.
Another method of weld-seam tracking is known (see German laid-open application No. 27 11 660) according to which the zone immediately around the welding spot is scanned by a television camera. Thus, there is obtained a two-dimensional image profile in the zone of the weld seam or close to it; the point through which the welding torch must pass is identified by known mathematical methods.
A drawback of this method lies in that the information received is not sufficient to ensure a sound weld seam, since it comes from a two-dimensional profile. Moreover, complex mechanical and electronic assemblies are used in the zone near the arc; this greatly reduces the reliability of the welding equipment.
There are further known seam tracking systems which measure the distance to the closely juxtaposed workpieces by the use of inductive, capacitive, microwave, pneumatic and other types of sensor.
A drawback of these systems lies in that the range of measurement is limited to several millimeters and their use in welding is rendered difficult because of the necessity to arrange them in the immediate vicinity of the arc where the temperature is high, molten metal is splashing continuously and be electromagnetic interferences are strong.
Still further known distance-measuring systems comprise interferometers using microwave, laser or other electromagnetic emissions to achieve a high accuracy of measurement.
A drawback of these latter systems as regards their utility for measuring the relative position of the torch and the workpieces to be welded lies in that, when a beam of electromagnetic energy passes from one workpiece to the other, there can occur a loss of signal which would disturb their operation.
Also known are systems for determining the position of the workpieces by means of a television camera giving an image of the weld zone from which definite geometric information can be derived. Such systems have the aforementioned drawback of necessitating a positioning of the camera in the vicinity of the arc, thereby exposing it to the harmful influences of the welding process. Moreover, the extraction of geometric information requires very complex logical computing devices with large memory, which makes them very expensive for practical purposes. Since the camera is disposed near the torch, the access of the latter to some types of seams is rendered difficult which limits the capacity of the welding machine, robot or automaton to produce such seams.